Inorganic pigments, and especially TiO.sub.2, are often incorporated in various polymers as opacifying agents. Also typically incorporated in such polymers are hindered amine light stabilizers ("HALS"); phenolic antioxidants (e.g., butylated hydroxytoluene, commonly referred to as "BHT", and related materials); flame retardants (e.g., Sb.sub.2 O.sub.3); and UV stabilizers.
However, a common problem with the use of such pigments (and especially TiO.sub.2 with such additives) is that the pigments can either promote the reaction of polymer additives with each other or react directly with one or more of the additives. These reactions lead to the formation of chromophores which usually reflect light in the yellow portion of the spectrum and hence produce a "yellowing" of the polymer. The aforementioned discoloration problem is especially apparent in white pigmented olefinic polymers.
While methods exist which somewhat inhibit the foregoing discoloration problem, they are deficient in one or more areas, e.g., they (1) can be expensive and/or complex to apply and/or (2) can cause deterioration of some polymer properties and/or cause processing problems such as difficulty in extruding the films or dispersing pigments in dry blends of the polymers.
The following information is disclosed which may be of interest to this invention:
U.S. Pat. No. 3,545,994 discloses masking the surface of TiO.sub.2 by coating it with various metal oxides.
U.S. Pat. No. 3,904,565 discloses a polymer color inhibitor consisting of a mixture of a fatty acid salt, a polyalkeneglycol ether, and thiobisphenol.
U.S. Pat. No. 3,673,146 discloses the use of triorganophosphites to inhibit polymer discoloration.
U.S. Pat. No. 4,054,522 discloses polymer color inhibitors consisting of esters of phosphorodihalous acids.
U.S. Pat. No. 4,357,170 discloses the use of a mixture of an organophosphate/alkanolamine addition product, and a polyol.
An article entitled "Discoloration of Pigmented Polyolefins" by D. A. Holtzen appeared in the April, 1977 issue of Plastics Engineering and discloses the use of zinc stearate to inhibit polymer discoloration.